This invention relates to a method for purifying alkoxysilanes which contain chlorine-based impurities. More specifically, the present invention relates to a method for purifying crude alkoxysilanes in which the total chlorine-based impurities are reduced to extremely low levels in the final purified alkoxysilanes.
Alkoxysilanes have recently found increasing application in electric and electronic fields; however, alkoxysilane purity must be extremely high in these applications. Due to this need for higher purity, the development of a purification method that affords a very pure alkoxysilane is a matter of some urgency.
Alkoxysilanes with the general formula R.sub.m Si(OR.sup.1).sub.4-m are produced by the liquid-phase or vapor-phase catalytic reaction of the corresponding chlorosilane with alcohol. The crude alkoxysilane product of this reaction contains hydrolyzable and non-hydrolyzable chlorine-based impurities. The crude alkoxysilane product containing such chlorine-based impurities has heretofore been purified by neutralization with a neutralization agent followed by distillation to separate the alkoxysilane from non-volatiles such as the neutralization salts. However, in this method, impurities based on non-hydrolyzable chlorine such as carbon-bonded chlorine, for example, chloromethyltrimethoxysilane, cannot be removed. Only hydrochloric acid and impurities based upon hydrolyzable chlorine, such as chlorosilane, etc., can form neutralization salts using any of anhydrous ammonia, amines, or basic compounds which contain metal, such as sodium, potassium or magnesium, etc., as the neutralization agent.
Chung and Hayes in Journal of Organometallic Chemistry, 265 (1984), pp.135-139, describe a method for the removal of impurities based upon non-hydrolyzable chlorine from alkoxysilanes. In the method of Chung and Hayes, alkoxysilane containing approximately 400 parts per million non-hydrolyzable chlorine is heated in the presence of either lithium aluminum hydride, tetramethylguanidine, magnesium oxide, dibutyltin dihydride or metallic sodium and then distilled. However, the problem with this method is that non-hydrolyzable chlorine present as an impurity in the alkoxysilane can be removed to less than 100 ppm only using metallic sodium. However, while the use of metallic sodium will give less than 100 ppm non-hydrolyzable chlorine in the alkoxysilane, the handling of metallic sodium poses a significant safety hazard for industrial operations.
As discussed above, prior purification methods suffer from problems in the removal of chlorine-based impurities. The objective of the present invention is to provide alkoxysilane which contains very low levels of chlorine-based impurities and, in particular, which has a low electrical conductivity, thus being useful in the electric and electronic fields, by virtue of a relatively simple method for freeing alkoxysilane of both hydrolyzable chlorine-based impurities and non-hydrolyzable chlorine-based impurities.
The method for purifying alkoxysilane according to the instant invention removes both hydrolyzable chlorine-based impurities and non-hydrolyzable chlorine-based impurities from alkoxysilane containing chlorine-based impurities by heating in the presence of acid clay or metal halide. Due to this novel feature, an alkoxysilane is readily produced which will contain extremely low chlorine-based impurity levels, levels not easily and simply attained by prior methods, and which will have a low electrical conductivity. Accordingly, it is quite useful in applications in electric and electronic fields, where a very pure and low-conductivity alkoxysilane is required, as well as in other prior applications of alkoxysilanes.